CHARLESTON, W.Va. -- A key corporate study used by federal health officials to set a screening level for "Crude MCHM" in the West Virginia American Water system actually tested a pure form of the material's main ingredient and might not account for potential toxicity of other components, documents and interviews with public health officials showed Friday."That is a huge problem," said Dr. Lynn Goldman, dean of the School of Public Health and Health Services at George Washington University.U.S. Centers for Disease Control and Prevention officials did not disclose the issue when they discussed their 1-part-per-million screening level for safe drinking water and did not respond Friday to repeated requests for comment.The issue, revealed when Eastman Chemical Co. made public its previously secret studies of the chemical it made and sold to Freedom Industries, raises more questions after last week's toxic leak that fouled drinking water for more than 300,000 West Virginians.Gov. Earl Ray Tomblin and other state officials, along with West Virginia American Water have been assuring residents -- except for expectant mothers -- that it's safe to use Elk River water, once the water company clears their particular neighborhood and home plumbing systems.Tomblin and his top public health officials have cited the screening number developed on an emergency basis by scientists at the CDC, who were working without any existing regulatory standards or published health guidance and dealing with very little data about the chemical.State and federal officials also pointed to water test results they say show a continuing decline in contaminant levels in the system. They also say the CDC safety figure was conservative and that the advisory for pregnant women was issued out of an "abundance of caution."The CDC's number was based largely on an April 1990 Eastman study in which rats were exposed to varying levels of 4-methylcyclohexanemethanol. The study concluded that a concentration of 100 milligrams per kilogram of the chemical was the "No Observable Adverse Effects Level," or NOAEL, for the material.CDC scientists used relatively standard risk-assessment procedures to translate that figure into a level of 1 part per million of the chemical they said likely would cause no adverse health effects in humans.However, emergency response and public health officials, as well as company documents, have said the chemical that actually leaked into the Elk River was Crude MCHM.An Eastman "material safety data sheet," or MSDS, for that chemical shows that 4-methylcyclohexanemethanol makes up 68 percent to 89 percent of Crude MCHM.The MSDS also shows, though, that Crude MCHM includes six other ingredients: 4-(methoxymethyl)cyclohexanemethanol, water, methyl 4-methylcyclohexanecarboxylate, dimethyl 1,4-cyclohexanedicarboxylate, methanol and 1,4-cyclohexanedimethanol.Evan Hansen, an environmental consultant with the Morgantown firm Downstream Strategies, has been wondering for several days about whether the health studies and water-sampling efforts included the other constituents of Crude MCHM.Posting his thoughts on Twitter, Hansen said the distinction between the two substances is important. "Are water tests in area being done for all constituents of Crude MCHM or just for 4-MCHM," Hansen posted.Writing on his group's blog Thursday night, Environmental Defense Fund biochemist Richard Denison said knowing that the Eastman study focused on a different chemical than was leaked "adds some additional uncertainty.""If other components besides MCHM present in the crude mixture are more or less toxic than MCHM, the mixture's toxicity would differ from that found for the pure material," Denison wrote.Goldman, who was assistant administrator for toxic substances in the Clinton administration's Environmental Protection Agency, agreed that the difference between the pure 4-MCHM and the "technical grade" of Crude MCHM are important."What is the actual concentration of the pure chemical in the technical grade, and then what are the other things in the technical grade and are they in the technical grade in high enough concentrations to be a concern," Goldman said in an interview Friday. "Question mark, question mark, question mark."Reach Ken Ward Jr. at email@example.com or 304-348-1702.